LinkedHashMap

Hash table and linked list implementation of the Map interface,
with predictable iteration order. This implementation differs from
HashMap in that it maintains a doubly-linked list running through
all of its entries. This linked list defines the iteration ordering,
which is normally the order in which keys were inserted into the map
(insertion-order). Note that insertion order is not affected
if a key is re-inserted into the map. (A key k is
reinserted into a map m if m.put(k, v) is invoked when
m.containsKey(k) would return true immediately prior to
the invocation.)

This implementation spares its clients from the unspecified, generally
chaotic ordering provided by HashMap (and Hashtable),
without incurring the increased cost associated with TreeMap. It
can be used to produce a copy of a map that has the same order as the
original, regardless of the original map's implementation:

void foo(Map m) {
Map copy = new LinkedHashMap(m);
...
}

This technique is particularly useful if a module takes a map on input,
copies it, and later returns results whose order is determined by that of
the copy. (Clients generally appreciate having things returned in the same
order they were presented.)

A special constructor is
provided to create a linked hash map whose order of iteration is the order
in which its entries were last accessed, from least-recently accessed to
most-recently (access-order). This kind of map is well-suited to
building LRU caches. Invoking the put, putIfAbsent,
get, getOrDefault, compute, computeIfAbsent,
computeIfPresent, or merge methods results
in an access to the corresponding entry (assuming it exists after the
invocation completes). The replace methods only result in an access
of the entry if the value is replaced. The putAll method generates one
entry access for each mapping in the specified map, in the order that
key-value mappings are provided by the specified map's entry set iterator.
No other methods generate entry accesses. In particular, operations
on collection-views do not affect the order of iteration of the
backing map. *

The removeEldestEntry(Map.Entry) method may be overridden to
impose a policy for removing stale mappings automatically when new mappings
are added to the map.

This class provides all of the optional Map operations, and
permits null elements. Like HashMap, it provides constant-time
performance for the basic operations (add, contains and
remove), assuming the hash function disperses elements
properly among the buckets. Performance is likely to be just slightly
below that of HashMap, due to the added expense of maintaining the
linked list, with one exception: Iteration over the collection-views
of a LinkedHashMap requires time proportional to the size
of the map, regardless of its capacity. Iteration over a HashMap
is likely to be more expensive, requiring time proportional to its
capacity.

A linked hash map has two parameters that affect its performance:
initial capacity and load factor. They are defined precisely
as for HashMap. Note, however, that the penalty for choosing an
excessively high value for initial capacity is less severe for this class
than for HashMap, as iteration times for this class are unaffected
by capacity.

Note that this implementation is not synchronized.
If multiple threads access a linked hash map concurrently, and at least
one of the threads modifies the map structurally, it must be
synchronized externally. This is typically accomplished by
synchronizing on some object that naturally encapsulates the map.
If no such object exists, the map should be "wrapped" using the
Collections.synchronizedMap
method. This is best done at creation time, to prevent accidental
unsynchronized access to the map:

Map m = Collections.synchronizedMap(new LinkedHashMap(...));

A structural modification is any operation that adds or deletes one or more
mappings or, in the case of access-ordered linked hash maps, affects
iteration order. In insertion-ordered linked hash maps, merely changing
the value associated with a key that is already contained in the map is not
a structural modification. In access-ordered linked hash maps,
merely querying the map with get is a structural
modification.)

The iterators returned by the iterator method of the collections
returned by all of this class's collection view methods are
fail-fast: if the map is structurally modified at any time after
the iterator is created, in any way except through the iterator's own
remove method, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent
modification, the iterator fails quickly and cleanly, rather than risking
arbitrary, non-deterministic behavior at an undetermined time in the future.

Note that the fail-fast behavior of an iterator cannot be guaranteed
as it is, generally speaking, impossible to make any hard guarantees in the
presence of unsynchronized concurrent modification. Fail-fast iterators
throw ConcurrentModificationException on a best-effort basis.
Therefore, it would be wrong to write a program that depended on this
exception for its correctness: the fail-fast behavior of iterators
should be used only to detect bugs.

The spliterators returned by the spliterator method of the collections
returned by all of this class's collection view methods are
late-binding,
fail-fast, and additionally report ORDERED.

public
LinkedHashMap()

Constructs an insertion-ordered LinkedHashMap instance with
the same mappings as the specified map. The LinkedHashMap
instance is created with a default load factor (0.75) and an initial
capacity sufficient to hold the mappings in the specified map.

Parameters

Returns

Performs the given action for each entry in this map until all entries
have been processed or the action throws an exception. Unless
otherwise specified by the implementing class, actions are performed in
the order of entry set iteration (if an iteration order is specified.)
Exceptions thrown by the action are relayed to the caller.

Parameters

Returns the value to which the specified key is mapped,
or null if this map contains no mapping for the key.

More formally, if this map contains a mapping from a key
k to a value v such that (key==null ? k==null :
key.equals(k)), then this method returns v; otherwise
it returns null. (There can be at most one such mapping.)

A return value of null does not necessarily
indicate that the map contains no mapping for the key; it's also
possible that the map explicitly maps the key to null.
The containsKey operation may be used to
distinguish these two cases.

Parameters

key

the key whose associated value is to be returned

Returns

the value to which the specified key is mapped, or
null if this map contains no mapping for the key

Replaces each entry's value with the result of invoking the given
function on that entry until all entries have been processed or the
function throws an exception. Exceptions thrown by the function are
relayed to the caller.